This invention relates to a detecting device for detecting a magnetic strip embedded in a sheet such as a check or other currency note.
Because of the excellent quality of copies produced by modern copying or printing machines, counterfeit notes, which are difficult to detect, have been easily made using these machines. Countermeasures against such counterfiets have been devised in many countries by embedding a metal strip in the note. A detecting device for automatically discriminating between genuine and counterfeit notes has been developed which detects the existence of the metal strip by utilizing eddy-current loss, as shown in the United Kingdom Patent Application No. 2017996. The eddy current loss is represented as PC=K(dB/dt), (B=magnetic flux density, K=electrical conductivity=1/electrical resistance). The electrical resistance value of non-magnetic metals is generally low (e.g., Al is 2.65 .mu..OMEGA.cm; Cu is 1.7 .mu..OMEGA.cm). Since the conductivity (K) between magnetic metals and non-magnetic metals greatly varies, the sensitivity to non-magnetic metals (e.g., aluminum) is high due to its high eddy current loss (Pc); the sensitivity to magnetic metals (e.g., iron), however, is very low due to the low eddy current loss (Pc). Therefore, the use of these prior art systems in detecting a magnetic metal strip has resulted in mis-detection or non-detection of the metal strip.
In order to detect magnetic strips embedded in paper notes, conventional inductive proximity detectors (see, e.g. U.K. Pat. No. 2,017,996) have been considered as an appropriate solution. Such detectors, however, are not capable of detecting a magnetic strip which is traveling rapidly in front of the detecting head, due to the very small diameter of the strip. As a result, the signal obtained has an insufficient level and may also contain noise signals.